U.S. patent application number 10/162149 was filed with the patent office on 2002-10-10 for laser optimized multimode fiber and method for use with laser and led sources and system employing same.
Invention is credited to Abbott, John S. III, Harshbarger, Douglas E..
Application Number | 20020146224 10/162149 |
Document ID | / |
Family ID | 27382582 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020146224 |
Kind Code |
A1 |
Abbott, John S. III ; et
al. |
October 10, 2002 |
Laser optimized multimode fiber and method for use with laser and
led sources and system employing same
Abstract
A multimode optical fiber having a first laser bandwidth greater
than 220 MHz.km in the 850 nm window, a second laser bandwidth
greater than 500 MHz.km in the 1300 nm window, a first OFL
bandwidth of at least 160 MHz.km in the 850 nm window, and a second
OFL bandwidth of at least 500 MHz.km in the 1300 nm window is
disclosed. The multimode fiber is capable of operating in
telecommunication systems employing both LED power sources and high
power laser sources. Methods of making and testing the multimode
optical fiber are also disclosed.
Inventors: |
Abbott, John S. III;
(Elmira, NY) ; Harshbarger, Douglas E.; (Corning,
NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
27382582 |
Appl. No.: |
10/162149 |
Filed: |
June 3, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10162149 |
Jun 3, 2002 |
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09501490 |
Feb 9, 2000 |
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6438303 |
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60121169 |
Feb 22, 1999 |
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60174722 |
Jan 6, 2000 |
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Current U.S.
Class: |
385/124 ;
385/123; 65/414; 65/435 |
Current CPC
Class: |
G02B 6/0288 20130101;
G02B 6/02214 20130101; C03B 2203/26 20130101; Y02P 40/57 20151101;
G02B 6/02 20130101; C03B 37/01413 20130101; C03B 2201/31 20130101;
C03B 2203/22 20130101; C03B 2203/31 20130101; C03B 2203/36
20130101 |
Class at
Publication: |
385/124 ;
385/123; 65/414; 65/435 |
International
Class: |
G02B 006/18; G02B
006/16; C03B 037/027 |
Claims
What is claimed is:
1. A method of forming a multimode optical fiber, said method
comprising the steps of: thermochemically reacting a silica
containing precursor reactant and at least one dopant reactant to
form soot; delivering the soot to a target in a manner sufficient
to produce a glass preform having specified characteristics; and
drawing the glass preform into a multimode optical fiber having a
62.5 .mu.m core region, and a cladding region bounding the core
region, and wherein said reacting step comprises selecting said
precursor reactant and said at least one dopant reactant according
to a soot deposition recipe sufficient to result in a multimode
optical fiber that exhibits a DMD profile, which when measured at a
wavelength of 1300 nm, includes a first average slope measured over
a first region from (r/a).sup.2=0.0 to 0.25, and a second average
slope measured over a second region from (r/a).sup.2=0.25 to 0.50,
and wherein the first average slope is greater than the second
average slope.
2. The method of claim 1 wherein said at least one dopant reactant
comprises germanium and wherein the reacting step comprises the
step of thermochemically reacting said germanium to form germania
containing soot, and wherein the delivering step comprises
selectively delivering the germania containing soot to a target in
sufficient quantities to result in the average slope over the first
region being at least 1.5 times greater than the average slope over
the second region.
3. The method of claim 2 wherein the reacting step comprises the
step of selecting a quantity of said germanium sufficient to result
in said multimode optical fiber exhibiting a change in DMD of at
least+0.3 nsec/km over the first region.
4. The method of claim 3 wherein the reacting step comprises the
step of selecting a quantity of said germanium sufficient to result
in said multimode optical fiber exhibiting a change in DMD of at
most+1.25 nsec/km over the first region.
5. The method of claim 1 wherein the reacting step comprises the
step of selecting a quantity of said germanium sufficient to result
said multimode optical fiber exhibiting a change in DMD of at
most+0.30 nsec/km over the second region.
6. The method of claim 1 wherein the reacting step comprises the
step of selecting a quantity of said germanium sufficient to result
said multimode optical fiber which includes a DMD profile having a
third average slope measured over a third region from
(r/a).sup.2=0.4 to 0.6, and wherein the change in DMD over the
third region is at most+0.20 nsec/km.
7. The method of claim 1 wherein said at least one dopant reactant
comprises germanium and wherein the reacting step comprises the
step of thermochemically reacting said germanium to form germania
containing soot, and wherein the delivering step comprises
selectively delivering the germania containing soot to a target in
sufficient quantities to result in the average slope over the first
region being at least 2 times greater than the average slope over
the second region.
8. The method of claim 7 wherein the reacting step comprises the
step of selecting a quantity of said germanium sufficient to result
in said multimode optical fiber exhibiting a change in DMD of at
least+0.40 nsec/km over the first region.
9. The method of claim 1 wherein said at least one dopant reactant
comprises germanium and wherein the reacting step comprises the
step of thermochemically reacting said germanium to form germania
containing soot, and wherein the delivering step comprises
selectively delivering the germania containing soot to a target in
sufficient quantities to result in the average slope over the first
region being at least 3 times greater than the average slope over
the second region.
10. The method of claim 9 wherein the reacting step comprises the
step of selecting a quantity of said germanium sufficient to result
in said multimode optical fiber exhibiting a change in DMD of at
least 0.5 nsec/km over the first region.
11. A multimode optical fiber comprising: a core having a diameter
of about 62.5 .mu.m; and a cladding bounding said core and having a
refractive index lower than said core refractive index, and wherein
said multimode optical fiber exhibits a DMD profile curve, which
when measured at a wavelength of 1300 nm, comprises a first region
having an average slope measured from (r/a).sup.2=0.0 to 0.25, and
a second region having an average slope measured from
(r/a).sup.2=0.25 to 0.50, and wherein the average slope of the
first region is greater than the average slope of the second
region.
12. The multimode optical fiber of claim 11 wherein the change in
DMD from (r/a).sup.2=0.0 to 0.25 is at least 1.5 times the change
in DMD from (r/a).sup.2=0.25 to 0.50.
13. The multimode optical fiber of claim 12 wherein the change in
DMD from (r/a).sup.2=0.0 to 0.25 is at least +0.3 nsec/km.
14. The multimode optical fiber of claim 13 wherein the change in
DMD from (r/a).sup.2=0.0 to 0.25 is at most +1.25 nsec/km.
15. The multimode optical fiber of claim 12 wherein the change in
DMD from (r/a).sup.2=0.25 to 0.50 is at most +0.30 nsec/km.
16. The multimode optical fiber of claim 11 wherein the DMD profile
includes a third slope measured from (r/a).sup.2=0.4 to 0.6, and
wherein the change in DMD from (r/a).sup.2=0.4 to 0.6 is at most
+0.20 nsec/km.
17. The multimode optical fiber of claim 11 wherein the change in
DMD from (r/a).sup.2=0.0 to 0.25 is at least 2 times the change in
DMD from (r/a).sup.2=0.25 to 0.50.
18. The multimode optical fiber of claim 17 wherein the change in
DMD from (r/a).sup.2=0.0 to 0.25 is at least 0.4 nsec/km.
19. The multimode optical fiber of claim 12 wherein the change in
DMD from (r/a).sup.2=0.0 to 0.25 is at least 3 times the change in
DMD from (r/a).sup.2=0.25 to 0.50.
20. The multimode optical fiber of claim 19 wherein the change in
DMD from (r/a).sup.2=0.0 to 0.25 is at least 0.5 nsec/km.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application hereby claims priority of U.S. Patent
Application Serial No. 60/121,169 filed on Feb. 22, 1999, and U.S.
Patent Application Serial No. 60/174,722 filed on Jan. 6, 2000, the
contents of which are relied upon and incorporated herein by
reference in its entirety, and the benefit of priority under 35
U.S.C. .sctn.120 is hereby claimed.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to a multimode
optical fiber and method for use with telecommunication systems
employing low data rates, as well as systems employing high data
rates, and more particularly, to a multimode optical fiber and
method optimized for applications designed for state of the art
laser sources, as well as common light emitting diode sources.
[0004] While the present invention is subject to a wide range of
applications, it is particularly well suited for use in
telecommunications systems designed to transmit data at rates equal
to and exceeding one gigabit/sec.
[0005] 2. Technical Background
[0006] The goal of the telecommunication industry is generally to
transmit greater amounts of information, over longer distances, in
shorter periods of time. Over time, it has been shown that this
objective is a moving target with no apparent end in sight. As the
number of systems users and frequency of system use increase,
demand for system resources increases as well.
[0007] Until recently, data networks have typically been served by
Local Area Networks (LANs) that employ relatively low date rates.
For this reason, Light Emitting Diodes (LEDs) have and continue to
be the most common light source in these applications. However, as
data rates begin to increase beyond the modulation capability of
LEDs, system protocols are migrating away from LEDs, and instead,
to laser sources. This migration is evidenced by the recent shift
toward systems capable of delivering information at rates equal to
and exceeding one (1) gigabit/sec.
[0008] While such transmission rates will greatly enhance the
capabilities of LANs, it does create an immediate concern for
system owners. Multimode optical fiber currently employed in
telecommunication systems is designed primarily for use with LED
sources and is generally not optimized for use with the lasers
envisioned to operate in systems designed to transmit information
at rates equal to or greater than one (1) gigabit/sec. Laser
sources place different demands on multimode fiber quality and
design, compared to LED sources. Historically, the index profile at
the core of multimode fibers has been tuned to produce high
bandwidth with LED sources, which tend to overfill the core. The
combination of the light intensity distribution from the LED source
input pulse and the index profile of the fiber produces an
overfilled modal weighting that results in an output pulse that has
a relatively smooth rise and fall. Although peaks or plateaus
resulting from small deviations from the ideal near-parabolic index
profile do occur, their magnitude does not impact system
performance at low data rates. In laser based systems, however, the
intensity distribution of the source concentrates its power near
the center of the multimode fiber. Consequently, small deviations
in the fiber profile can produce significant perturbations in the
impulse rise and fall, which can have a large effect on system
performance. This effect can manifest itself in the form of
excessively low bandwidth, as excessively high temporal jitter, or
both. Although it is possible to correct these deficiencies to some
degree by changing the launch condition of the source, such as the
offset launch mode conditioning patch cord or the laser beam
expander, this is typically not a practical solution for system
owners.
[0009] A typical campus layout for a LAN system is designed to meet
certain specified link lengths. The standard for the campus
backbone (which travels between buildings) typically has a link
length of up to about 2 km. The building backbone or riser (which
travels between floors of a building)typically has a link length of
up to about 500 meters. The horizontal link length (which travels
between offices on a floor of a building) typically has a link
length of up to about 100 meters. Older and current LAN technology,
such as 10 Megabit Ethernet, can achieve a 2 km link length
transmission with standard grade multimode optical fiber. However,
next generation systems capable of gigabit/sec. and higher
transmission rates cannot achieve all of these link lengths with
standard multimode fiber presently available. In the 850 nm window,
standard multimode fiber is limited to a link length of
approximately 220 meters. In the 1300 nm window, standard grade
fiber is limited to a link length of only about 550 meters.
Accordingly, present technology only enables, at most, coverage for
about two of the three campus link lengths. To fully enable a LAN
for gigabit/sec. transmission rates, a multimode fiber capable of
transmitting information over each of the three link lengths is
necessary.
[0010] As used herein, overfilled (OFL) bandwidth is defined as the
bandwidth using the standard measurement technique described in
EIA/TIA 455-51 FOTP-51A, "Pulse Distortion Measurement of Multimode
Glass Optical Fiber Information Transmission Capacity", with launch
conditions defined by EIA/TIA 455-54A FOTP-54"Mode Scrambler
Requirements for Overfilled Launching Conditions to Multimode
Fibers".
[0011] As used herein, laser bandwidth is defined as and measured
using the standard measurement technique described in EIA/TIA
455-51A FOTP-51 and either of the following two launch conditions
methods. Method (a) is used to determine the 3 dB bandwidth at
1300, and method (b) is used to determine the 3 dB bandwidth at 850
nm. Method (a), which is used to determine the 3 dB laser bandwidth
at 1300 nm, utilizes a 4 nm RMS spectral width 1300 nm laser with a
category 5 coupled power ratio launch modified by connection of a 2
meter, standard step index, single-mode fiber, patch-cord wrapped
twice around a 50 mm diameter mandrel. The launch condition is
further modified by mechanically offsetting the central axis of the
singlemode fiber from that of the multimode fiber in such a manner
that a 4 um lateral offset between the central axis of the core of
the single mode fiber patch-cord and the multimode fiber under test
is created. Note: category 5 coupled power ratio is described in
and measured using procedures in TIA/EIA 526-14A OFSTP 14 appendix
A "Optical Power Loss Measurements of Installed Multimode Fiber
Cable Plant. Method (b), which is used to determine the 3 dB laser
bandwidth at 850 nm, utilizes a 0.85 nm RMS spectral width 850 nm
OFL launch condition, as described in EIA/TIA 455-54A FOTP 54,
connected to a 1 meter length of a specially designed multimode
fiber having a 0.208 numerical aperture and a graded index profile
with and alpha of 2. Such a fiber can be created by drawing down a
standard 50 .mu.m diameter core multimode fiber having a 1.3 index
of refraction delta (delta=n.sub.o.sup.2-n.sub.c.sup.2/2n.sub-
.on.sub.c, where n.sub.o=the index of refraction of the core and
n.sub.c=the index of refraction of the cladding) to a 23.5 .mu.m
diameter core.
[0012] Today, in order to increase distance, manufacturers
typically shift bandwidth between two wavelength windows by
changing the shape of the refractive index profile. Depending upon
the changes made, the result is either high OFL bandwidth at the
850 nm window with low OFL bandwidth at the 1300 nm window, or low
OFL bandwidth at the 850 nm with high OFL bandwidth at the 1300 nm
window. For example, for a standard 2% Delta 62.5 um FDD-type
fiber, the refractive index profile can be adjusted to result in
OFL bandwidth of 100 OMHz.km at 850 nm and 300 MHz.km at 1300 nm,
or it can be adjusted to result in OFL bandwidth of 250 MHz.km at
850 nm and 400 OMHz.km at 1300 nm. With such multimode optical
waveguide fibers having standard "alpha" profiles, however, it is
not possible to achieve an OFL bandwidth of 1000 MHz.km at 850 nm
and 4000 MHz.km at 1300 nm. More typically, manufacturing
tolerances would allow 850 nm/1300 nm OFL bandwidths of 600
MHz.km/300 MHz.km or 200 MHz.km/1000 MHz.km but not 600 MHz.km/1000
MHz.km.
[0013] There is a disconnect, however, between these historical
bandwidth shifts, and what is necessary for gigabit/sec.
transmission rates. Because high speed lasers are the standard
light source for LANs designed to deliver information at rates
exceeding a gigabit/sec., a multimode optical fiber having
increased bandwidth at both the 850 nm and 1300 nm window is
desired.
[0014] Moreover, because such LANs are in their infancy, all of the
system components necessary to meet and/or exceed transmission
rates of one gigabit/sec. are not yet fully reduced or practiced,
optimized, and/or tested. For these reasons, it is not practical to
replace existing LAN systems with a new LAN system speculatively
designed to meet or exceed such high data rates. While it may be
possible to achieve this result, it will likely not be the
preferred or optimal solution, as following such a course of action
will likely result in costly upgrades to the system and potentially
a rework of the entire system.
SUMMARY OF THE INVENTION
[0015] The present invention is directed to a multimode optical
fiber that is optimized for high speed laser sources capable of
1.0, 2.5, and 10 gigabit per second data transmission while
exceeding the link length requirements discussed above. Moreover,
the same multimode optical fiber maintains sufficiently high OFL
bandwidth to support the transmission of information with the 1300
nm and 850 nm LED sources presently used in LAN systems. Such a
multimode optical fiber will enable current LAN system owners to
maintain their present LED based LAN systems, while at the same
time enable them to easily transfer to a "Gigabit Ethernet System"
without having to undertake a costly multimode fiber upgrade. As
used herein, "Gigabit Ethernet System" is defined as a
telecommunication system, such as a LAN, which is capable of
transmitting data at rates equal to and/or exceeding one (1)
gigabit/sec.
[0016] Accordingly, one aspect of the present invention relates a
multimode fiber having a first laser bandwidth greater than 220
MHz.km in the 850 nm window, a second laser bandwidth greater than
500 MHz.km in the 1300 nm window, a first OFL bandwidth of at least
160 MHz.km in the 850 nm window, and a second OFL bandwidth of at
least 500 MHz.km in the 1300 nm window. Such a multimode optical
fiber has a variety of uses in the telecommunication industry, and
is particularly well suited for use in telecommunication systems
employing high speed laser sources. Such a fiber has the added
benefit of providing sufficient OFL bandwidth for LED sources
presently used in LAN systems.
[0017] In another aspect, the invention is directed to a multimode
transmission system capable of transmitting data at rates equal to
and exceeding one gigabit/sec. The multimode transmission system
includes a laser source which transmits at least one gigabit/second
of information, and a multimode optical fiber communicating with
the laser source. The multimode optical fiber has a first laser
bandwidth of at least 385 MHz.km in the 850 nm window which is
capable of carrying the information at least 500 meters. The
multimode optical fiber also has a second laser bandwidth of at
least 746 MHz.km in the 1300 nm window for carrying the information
at least 1000 meters. In addition, the multimode optical fiber
includes first and second OFL bandwidths sufficiently high to be
used with 850 nm and 1300 nm LED sources.
[0018] Another aspect of the present invention relates to a
multimode optical fiber having a 62.5 .mu.m core, and a cladding
bounding the core. The cladding has a refractive index lower than
the refractive index of the core, and the multimode optical fiber
exhibits a DMD profile, which when measured at a wavelength of 1300
nm, includes a first region having an average slope measured from
(r/a).sup.2=0.0 to 0.25, and a second slope region having an
average slope measured from (r/a).sup.2=0.25 to 0.50. The slope of
the first region is preferably greater than the slope of the second
region. More preferably, the slope of the first region is greater
than 1.5 times the slope of the second region.
[0019] In a further aspect, the present invention is directed to a
method of forming a multimode optical fiber. The method includes
the steps of thermochemically reacting a silica containing
precursor reactant and at least one dopant reactant to form soot,
and delivering the soot to a target in a manner sufficient to
produce a glass preform having specified characteristics. The glass
preform is drawn into a multimode optical fiber having a 62.5 .mu.m
core region and a cladding region bounding the core region. The
reacting step includes selecting a precursor reactant and a dopant
reactant according to a soot deposition recipe sufficient to result
in a multimode optical fiber which exhibits a DMD profile, which
when measured at a wavelength of 1300 nm, has a first average slope
measured over a first region from (r/a).sup.2=0.0 to 0.25, and a
second average slope measured over a second region from
(r/a).sup.20.25 to 0.50, the first average slope being greater than
the second average slope.
[0020] The multimode optical fiber of the present invention results
in a number of advantages over other multimode optical fibers known
in the art. One such advantage is that the multimode optical fiber
of the present invention is fully compatible for use with high
speed laser sources, as well as LED sources. Accordingly, the
multimode optical fiber of the present invention can be used with
conventional local area networks employing LED sources, and can be
used with Gigabit Ethernet Systems, which employ high speed laser
sources.
[0021] In addition, the multimode optical fiber of the present
invention eliminates the need for costly mode conditioning patch
cords often used to enable operation in the 1300 nm operating
window for Gigabit Ethernet System protocol. For many multimode
optical fibers, a mode conditioning patch cord is used to move
power away from the center of the multimode fiber in order to avoid
center line profile defects which typically result from some
manufacturing processes. Because the preferred multimode optical
fiber of the present invention is manufactured using the Outside
Vapor Deposition process (OVD), the preferred multimode optical
fiber of the present invention has reduced centerline profile
defects. Accordingly, a mode conditioning patch cord is no longer
needed to enable operation in the 1300 nm operating window of the
preferred fiber of the present invention, thus allowing for
on-center launch or slightly off-set due to loose connector
tolerances, resulting in ease of installation and use.
[0022] Moreover, the multimode optical fiber of the present
invention optimizes laser performance with a variety of laser
sources, such as, but not limited to, 780 nm Fabry-Perot lasers,
850 nm Vertical Cavity Surface Emitting Lasers (VCSELs), 1300 nm
Fabry-Perot lasers, and low cost 1300 nm transmitters envisioned
for the future. The multimode optical fiber of the present
invention is also designed to support operation at 2.5 and 10
gigabits/second over significant link lengths when used with high
performance lasers in more advanced telecommunication systems.
[0023] Additional features and advantages of the invention will be
set forth in the detailed description which follows, and in part
will be readily apparent to those skilled in the art from that
description or recognized by practicing the invention as described
herein, including the detailed description which follows, the
claims, as well as the appended drawings.
[0024] It is to be understood that both the foregoing general
description and the following detailed description are merely
exemplary of the invention, and are intended to provide an overview
or framework for understanding the nature and character of the
invention as it is claimed. The accompanying drawings are included
to provide a further understanding of the invention, and are
incorporated in and constitute a part of this specification. The
drawings illustrate various embodiments of the invention, and
together with the description serve to explain the principles and
operation of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a perspective view of a preferred embodiment of a
multimode optical fiber of the present invention.
[0026] FIG. 2 is a DMD profile curve of the multimode optical fiber
of FIG. 1 measured at 1300 nm.
[0027] FIG. 3 is a DMD profile curve of the multimode optical fiber
of FIG. 1 measured at 850 nm.
[0028] FIG. 4 is a DMD profile curve of a second preferred
embodiment of a multimode optical fiber of the present invention
measured at 1300 nm.
[0029] FIG. 5 is a graph showing the DMD profile curve of the
multimode optical fiber of FIG. 1, and the DMD profile curve for a
second preferred multimode optical fiber measured at 1300 nm.
[0030] FIG. 6 shows the bandwidth of the optical fiber of FIG. 1
for a variety of laser sources.
[0031] FIG. 7 is the refractive index profile curve of the first
preferred embodiment of the multimode optical fiber of the present
invention, which has the DMD profile of FIG. 2.
[0032] FIG. 8 is the refractive index profile curve of the second
preferred embodiment of the multimode optical fiber of the present
invention, which has the DMD profile of FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] A refractive index profile for a multimode optical fiber is
disclosed, which is optimized both for applications using state of
the art laser sources, as well as the more common LED sources.
Alpha index of refraction profiles describe a profile shape which
may vary continuously with radius. In the present invention, the
refractive index profile preferably includes at least two regions
having at least "alpha" exponents, commonly referred to by the
symbol (.alpha.), such that the index profile changes smoothly from
an alpha or alphas optimized for one or more laser sources (at one
or more wavelengths) near the center of the profile to an alpha or
alphas optimized for LEDs (at one or more wavelengths) near the
outside of the profile. A multimode optical fiber having such an
index profile extends both distance and data rate capability beyond
that documented for telecommunication systems capable of delivering
information at rates equal to and exceeding one (1) gigabit/sec.
Because laser sources have smaller "spots" than LEDs, it has been
found that the outer portion of the profile can be optimized
according to OFL bandwidth requirements (typically 160-200 MHz.km
at 850 nm and 500+ MHz.km at 1300 nm, for multimode fibers having
62.5 um cores), while simultaneously optimizing the inner portion
of the profile for laser bandwidth requirements and source
characteristics. It is believed that this is the first profile
which is simultaneously optimized for both large spot LEDs and
small spot lasers at both the 1300 nm and 850 nm windows. Because
the 1300 nm laser spot is even smaller than that of short
wavelength (SX) laser sources, the inner profile requirements are
preferably determined by the SX bandwidth requirements. It has been
found that high laser bandwidth at both short wavelength (for
example, with selected 780 nm CD lasers or 850 nm VCSELs), and long
wavelength (for example, with 1300 nm or 1500 nm Fabry-Perot single
mode lasers) can be achieved when the inner profile is correctly
optimized.
[0034] An important characteristic of the optimized index profile
is that it provides high 1300 nm OFL bandwidth with LED sources so
that the adjustment to the overall profile to achieve superior
performance with lasers is small and/or in areas of the profile not
affecting OFL bandwidth performance. This also requires that
alpha(r) be a smooth function of r, without abrupt transitions.
[0035] The present invention is directed to a multimode optical
fiber having an index profile specifically designed to provide high
bandwidth and low temporal jitter with typical short wavelength
(for example, 780, 850, or 980 nm) lasers and long wavelength (for
example, 1300 nm or 1500 nm) lasers while maintaining sufficiently
high bandwidth and low jitter when used with legacy, 1300 nm and
850 nm LED sources.
[0036] The index profile of the multimode optical fiber of the
present invention can be described in a number of ways. First, in a
multimode fiber with M modes, the output pulse can be described as
P.sub.out(t)=.SIGMA.P.sub.m.delta.(.tau..sub.m-.tau..sub.ave),
where the m.sup.th mode has relative power P.sub.m and mode delay
.tau..sub.m relative to the average
.tau..sub.ave=.SIGMA.P.sub.m.tau.m/.SIGMA.P.sub.m- . The OFL or
laser bandwidth is determined from the amplitude of the Fourier
transform of P.sub.out(t) and is optimized if all .tau..sub.m are
equal.
[0037] The mode delays .tau..sub.m are determined by the index
profile and the wavelength of operation. The modal power P.sub.m
depends on the characteristics of the source (the specific laser,
LED, etc.). The multimode fibers of the present invention are
preferably designed to meet the OFL or laser bandwidth requirements
for a majority of, and most preferably all, of the commonly used
sources. For example, the fiber requirements might be that the OFL
bandwidth be greater than 160 MHz.km and 500 MHz.km with 850 nm and
1300 nm LED sources, respectively, and that the laser bandwidth be
greater than 385 MHz.km and 746 MHz.km with 850 nm VCSEL and 1300
nm Fabry-Perot laser sources, respectively.
[0038] A second way of describing the fiber's index profile relates
to direct measurement of the index of refraction or the germania
content of the core. Typical multimode fibers are designed to a
have an index of refraction that varies as a function of radial
position and is proportional to the germania content. This index
profile, n(r), is described by the following function:
For r<a, n(r)=n.sub.1(1-2.DELTA.(r/a).sup.g).sup.0.5
[0039] where n.sub.1 is the index value at the center of the core,
r is the radial position, a is the radius of the core clad
interface, g is the profile shape parameter, and .DELTA. is defined
as:
.DELTA.(n.sub.1.sup.2-n.sub.0.sup.2)/2n.sub.1.sup.2
[0040] where n.sub.0 is the index value at the core-clad interface.
This profile description is common in the literature with the
exponent "g" frequently being denoted as alpha (.alpha.). Those
skilled in the art use both terms interchangeably without
confusion.
[0041] For purposes of the present invention the index profile is
defined as follows:
For 0<r<a,
n(r)=n.sub.1(1-2.DELTA.(r/a).sup.g(r)).sup.0.05
[0042] Here g(r) is a profile shape parameter which changes
continuously with radius so that the OFL and laser bandwidth
objectives described above in the first method of describing the
index profile are met. Roughly speaking, the relative power of
modes near the center is greater for the laser sources than for the
LED sources, and greater for the long wavelength lasers (for
example, 1300 nm Fabry-Perot laser) than for the short wavelength
laser sources (for example, the typical 850 nm VCSEL sources).
Thus, heuristically g(r) might vary from being optimized at 1300 nm
at the very center, to being optimized for 850 nm at intermediate
radii, and to being optimized at 1300 nm for larger radii. In
practice, it is adequate for g(r) to vary from a larger value
(equalizing mode delays closer to 780-850 nm) near the center to a
lower value (equalizing closer to 1300 nm) at the outside. In
practice g(r) never intentionally passes below the value
appropriate for 1300 nm. It is important for the OFL bandwidth that
g(r) vary smoothly and continuously.
[0043] Such an index profile with a varying g(r) can perhaps be
visualized most easily with a third method for describing index
profiles. This method uses what are known to those skilled in the
art, as Differential Mode Delay (DMD) measurements. The method,
briefly described, involves scanning a pulse from a single mode
optical fiber radially across the multimode fiber core, and
measuring the output pulse and mean delay time for pulses launched
at different outset positions with respect to the core of the
multimode fiber. The pulse delays are plotted as a function of
radial position, and the local slope of the DMD vs. (r/a).sup.2,
where "r" is defined as the radial offset of the single mode fiber
relative to the center of the multimode core (i.e. the distance
between the axial center of the single mode fiber and the axial
center of the multimode core), and "a" is defined as the radius of
the core of the multimode fiber, approximates the index profile
parameter g(r). The local slope of the DMD vs (r/a).sup.2 curve is
proportional to the local g(r) error relative to the optimum g (or
alpha) for the given wavelength and Delta of the multimode optical
fiber. The relationships between the DMD, the index error, and the
"alpha error" is known to those skilled in the art, and is
described in the following references. Reference is made to
Marcuse, Principles of Optical Fiber Measurements, pp. 255-310,
(Academic Press, 1981), which is incorporated herein by reference
as though fully set forth in its entirety, and to Olshansky, R.,
"Propagation in Glass Optical Waveguides," Rev. Mod. Phys., Vol.
51, No. 2, April 1979, pp. 341-367, which is incorporated herein by
reference as though fully set forth in its entirety, for a more
detailed explanation of DMD measurements and techniques. In
accordance with a preferred embodiment of the present invention,
the OFL and laser bandwidth of a number of fibers with different
refractive index profiles (and therefore DMDs) are measured, and
fibers which achieve high bandwidth with both laser and LED sources
are identified. The DMD of these optimum fibers characterize the
desired or target profile for duplication in additional multimode
optical fibers. This empirical procedure using the DMD does not
characterize the P.sub.m of the different sources. Rather, it
serves to characterize the fiber which works with the sources.
[0044] A key aspect of the present invention is that laser
intensity distributions are generally much smaller than LEDs. For
that reason, among others, it is possible to optimize the fiber
index profile for both laser and LED operation. In accordance with
one embodiment of the present invention, the outer portion of the
index profile is optimized for 1300 nm LED, thereby ensuring good
performance, i.e. OFL bandwidth greater than 500 MHz.km, for legacy
systems. The inner portion of the index profile is optimized to
provide more equal laser bandwidth at 1300 nm and 850 nm. By
augmenting this design with manufacturing techniques that ensure a
smooth index change, a multimode optical fiber having high laser
bandwidth and low jitter for lasers of both wavelengths can be
repeatedly manufactured.
[0045] Reference will now be made in detail to the present
preferred embodiments of the invention, examples of which are
illustrated in the accompanying drawings. Wherever possible, the
same reference numbers will be used throughout the drawings to
refer to the same or like parts. An exemplary embodiment of the
multimode optical fiber of the present invention is shown in FIG.
1, and is designated generally throughout by reference numeral
10.
[0046] Preferred multimode optical fiber 10 is a 62.5 .mu.m
multimode optical fiber optimized to have a first laser bandwidth
greater than 220 MHz.km at 850 nm, and a second laser bandwidth
greater than 500 MHz.km at 1300 nm. It will be understood by those
skilled in the art, however, that multimode fibers in accordance
with the present invention have been made which likely have
similarly large bandwidths across the 850 and 1300 nm operating
windows, i.e., between about 810 nm and 890 nm, more preferably 830
nm and 870 nm, and between about 1260 nm and 1340 nm, more
preferably between about 1280 nm and 1320 nm.
[0047] In addition, preferred multimode optical fiber 10 includes a
first OFL bandwidth of at least 160 MHz.km in the 850 nm window,
and a second OFL bandwidth of at least 500 MHz.km in the 1300 nm
window. More preferably, however, multimode optical fiber 10 has a
62.51 .mu.m core 12 and is designed for a minimum laser bandwidth
of 385 MHz.km at 850 nm, and a minimum laser bandwidth of 746
MHz.km at 1300 nm. It should be noted that the 1300 nm laser
bandwidth mentioned above and described throughout the entirety of
this specification, should preferably be measured with a 1300 nm
laser meant for use with standard single mode fiber. It is
presently believed by many of those skilled in the art that
telecommunication systems capable of delivering data at rates equal
to or exceeding one gigabit/sec. will require a mode conditioning
patch cord to offset the laser launch at 1300 nm. For the multimode
optical fiber of the present invention, however, the laser launch
at 1300 nm is measured with the majority of the power being
launched along the central axis of the multimode fiber. This
obviates the need for such mode conditioning patch cords, thereby
reducing system implementation, cost, and complexity. For a
multimode optical fiber having a 50 .mu.m core (not shown), the
minimum laser bandwidth is preferably 500 MHz.km in the short
wavelength window and 1684 MHz.km in the long wavelength window.
When employed in a multimode transmission system employing high
speed laser sources, such as a telecommunication system designed to
transmit data at a rate of at least one (1) gigabit/sec., multimode
optical fiber 10 having the 62.5 .mu.m core 12 can carry at least
one gigabit/sec of information over a link length of at least 500 m
in the short wavelength, and over a link length of 1000 m in the
long wavelength. These distances are increased to link lengths of
over 600 m and 2000 m, respectively, for a 50 .mu.m core multimode
optical fiber. Those skilled in the art, however, will recognize
that preferred multimode optical fiber 10 is not limited to the one
gigabit/sec transmission rate. Rather, the present invention is
capable of data rate transmission in excess of ten (10)
gigabits/sec over significant link lengths. DMD measurement curves
indicative of 62.5 .mu.m core multimode optical fibers having
properties sufficient to meet the above-described operating
parameters are depicted in FIGS. 2 through 5.
[0048] FIG. 2 shows a DMD measurement curve 20 of a multimode
optical fiber 10 made in accordance with the present invention. The
DMD measurements of multimode optical fiber 10 were taken at 1300
nm using a standard pulse-based measurement technique similar to
that described in Marcuse, Principles of Optical Fiber
Measurements, pp. 255-310, (Academic Press, 1981), and Olshansky,
R., "Propagation in Glass Optical Waveguides," Rev. Mod. Phys.,
Vol. 51, No. 2, April 1979, pp. 341-367, which have been
incorporated herein by reference. In a region where the 1300 nm DMD
measurement curve slopes up the index profile is essentially
optimized for a wavelength less than 1300 nm, and in a region where
the DMD curve slopes down the index profile is essentially
optimized for a wavelength greater than 1300 nm. In the region
where the DMD curve is nearly flat the index profile is essentially
optimized for 1300 nm.
[0049] A DMD measurement curve 30 of multimode optical fiber 10,
measured at 850 nm using a commercially available Photon-Kinetics
Model 2500 Optical Fiber Measurement Bench, is depicted in FIG. 3.
Again, in the region where the DMD curve is slightly rising, the
index profile is optimized for a wavelength slightly less than 850
nm, and in the region where the DMD curve slopes down, it indicates
an index profile is optimized for a wavelength greater than 850
nm.
[0050] A DMD profile 40 measured at 1300 nm of a second preferred
multimode optical fiber (not shown), is depicted in FIG. 4.
Although DMD profile 40 differs slightly from DMD profile 20, it
also describes multimode optical fibers having properties
sufficient to meet the desired operating parameters for a multimode
optical fiber having a 62.5 .mu.m or 50 um core.
[0051] DMD profiles 20 and 40 are both shown in the same graph in
FIG. 5 measured at 1300 nm. The plots have each been shifted so
that they agree at a common point where the slopes are similar
(rather than (r/a).sup.2=0), and this point is arbitrarily defined
as zero (0) delay. Broadly speaking, when measured at a wavelength
of 1300 nm, a target DMD profile includes a first region having an
average slope measured from (r/a).sup.2=0.0 to 0.25, and a second
region having an average slope measured from (r/a).sup.2=0.25 to
0.50, the slope of the first region being greater than the slope of
the second region. Said differently, the target DMD profile is not
linear. More preferably, the slope of the first region is at least
1.5 times greater than the slope of the second region. Most
preferably, the target DMD profile includes a third region having
an average slope measured from (r/a).sup.2=0.4 to 0.6, in which the
change in DMD from (r/a).sup.2=0.4 to 0.6 is at most +0.20
nsec/km.
[0052] The preferred method of forming a multimode optical fiber in
accordance with the present invention and having the
above-described target DMD profile includes the steps of
thermochemically reacting a silica containing precursor reactant
and at least one dopant reactant to form soot, delivering the soot
to a target in a manner sufficient to produce a glass preform
having specified characteristics, and drawing the glass preform
into a multimode optical fiber having a 62.5 .mu.m or 50 um core
region. The reacting step further includes selecting the precursor
reactant and at least one dopant reactant according to a soot
deposition recipe sufficient to result in a multimode optical fiber
that exhibits the characteristics of the target DMD profile. In a
preferred embodiment, the soot deposition recipe includes the
required proportions of SiCl.sub.4 and GeCl.sub.4 which result in a
multimode optical fiber meeting the requirements of the desired
target profile. When measured at a wavelength of 1300 nm, Such a
multimode optical fiber will have a first average slope measured
over a first region from (r/a).sup.2=0.0 to 0.25, and a second
average slope measured over a second region from (r/a).sup.2=0.25
to 0.50, with the first average slope being greater than the second
average slope. It will be understood, however, that the present
invention is not limited to SiCl.sub.4 and GeCl.sub.4.
[0053] FIG. 7 shows the substantially parabolic refractive index
profile curve of the first preferred embodiment of the multimode
optical fiber of the present invention (the same fiber that
exhibits the DMD profile curve of FIG. 2 and 3). FIG. 8 shows the
substantially parabolic refractive index profile curve of the
second preferred embodiment of the multimode optical fiber of the
present invention (the same fiber that exhibits the DMD profile of
FIG. 4). Although these figures are not needed for practicing the
present invention, as described above, they clearly demonstrate the
benefit of the DMD measurement techniques used in accordance with
the present invention. With the exception of the slight differences
in the refractive index profile perturbations at the peak regions
of the refractive index profiles depicted in FIGS. 7 and 8, the
other regions of the refractive index profiles are strikingly
similar for both the first and second preferred embodiments of the
multimode optical fibers of the present invention.
[0054] While it is not specifically described herein, a multimode
optical fiber having a 50.0 .mu.m core can be similarly formed. It
will be understood by those skilled in the art that the target DMD
profile for such a multimode optical fiber will differ from the
target DMD profile of a multimode optical fiber having a 62.5 .mu.m
core as described above. Thus, the soot deposition recipe will
differ as well. It will be further understood that a target DMD
profile can be described by defining the regions of slope as a
first region from (r/a).sup.2=0.0 to 0.2, and a second region from
(r/a).sup.2=0.2 to 0.4.
EXAMPLE
[0055] The invention will be further clarified by the following
example which is intended to be exemplary of the invention.
Example 1
[0056] One method of testing the performance of the laser optimized
multimode fiber is to manufacture a fiber having the desired DMD
characteristics and test it with a variety of laser sources. The
results of such testing are shown in FIG. 6.
[0057] The `effective` bandwidth (MHz.km) of the multimode optical
fiber characterized by the DMD profile depicted in FIGS. 2-3 and 7
is shown in FIG. 6 for a variety of 780 to 850 nm Gigabit Ethernet
System lasers. The fiber's overfilled (OFL) bandwidth, measured
using the standard measurement and launch techniques referenced
earlier in this application, was 288 MHz.km at 850 nm and 1054
MHz.km at 1300 nm. The fiber's laser bandwidth, measured using the
standard measurement and launch techniques referenced earlier in
this application was 930 MHz.km at 850 nm (using the patchcord with
a 23.5 um diameter core as well as a 850 nm source laser with an
RMS spectral width less than 0.85 nm, as described earlier) and
2028 MHz.km at 1300 nm (using a Fabry-Perot laser typical for
single mode fiber applications and a patchcord ensuring a launch
offset 4 um from the center of the core). The `effective`
bandwidths shown in FIG. 6 for various Gigabit Ethernet System
laser sources are measured with the same measurement technique as
the defined 850 nm laser bandwidth with a 23.5 um patchcord, but
with a launch condition that varies with each individual Gigabit
Ethernet System laser because each laser has a different
distribution of power, both in the near field and in the far field.
This demonstrates that large bandwidths may be exhibited using the
fiber of the present invention together with a large variety of
laser launches. The measured laser bandwidth with the defined
launch (930 MHz.km) is approximately the same as obtained with a
number of actual Gigabit Ethernet Systemlasers. The short
wavelength Gigabit Ethernet System laser bandwidths are all clearly
superior to the 850 nm OFL bandwidth of 288 MHz.km and in the range
required to significantly extend Gigabit Ethernet System link
lengths. In addition, the 1300 nm laser bandwidth measured using a
1300 nm Fabry-Perot laser with a 4 um offset was more than double
that of the 1300 nm OFL bandwidth.
Example 2
[0058] As a second example, the fiber whose measured DMD is in FIG.
4 and whose measured index profile is in FIG. 8 was tested for OFL
bandwidth, for `defined` laser bandwidth using the 23.5 um
patchcord at 850 nm and the 4 um offset at 1300 nm, and for
`effective` bandwidth with a set of 13 Gigabit Ethernet System
lasers. The standard OFL bandwidth was measured at 564 MHz.km at
850 nm and 560 MHz.km at 1300 nm. The `defined` laser bandwidth at
850 nm using the patchcord with a 23.5 um diameter core was 826
MHz.km, while at 1300 nm the laser bandwidth defined by using a
Fabry-Perot laser with a 4 um offset had a value of 5279 MHz.km.
The `effective` bandwidths measured with 13 Gigabit Ethernet System
lasers at 850 nm or 780 nm were as follows: 1214, 886, 880,876,
792, 786, 754, 726, 614, 394, 376, 434 and 472 MHz.km. Again, the
defined laser launch for 850 nm with a patchcord with a 23.5 um
diameter core yields a bandwidth which approximates the `effective`
bandwidth seen with a number of actual Gigabit Ethernet laser
sources.
[0059] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention
without departing from the spirit and scope of the invention. Thus,
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *